Hollinger Corp. 
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\ 

UNITED STATES DEPARTMENT OF AGRICULTURE 



1 BULLETIN No. 916 



Contribution from the Bareaa of Plant Industry 
WM. A. TAYLOR, Chief 




4\-^^W'(U 




ji\^?^^§*Lnu 



Washington, D. C. 



PROFESSIONAL PAPER 



June 13, 1921 



FREEZING INJURY TO POTATOES WHEN 
UNDERCOOLED.^ 

By R. C.^ Wright, Physiologist, and George F. Taylor, Chemical Lahoratoriany 
Office of Horticultural and Pomological Investigations. 



CONTENTS. 



Page. 

Scope of the investigations 1 

Physiology of the freezing process 2 

Plan of the work 4 



Page. 
Inoculation of uudeicooled potatoes- 7 

Summary 14 

Literature cited 15 



SCOPE OF THE INVESTIGATIONS. 

Each year the loss to growers, shippers, and carriers of potatoes 
due to freezing reaches an enormous figure. This is particularly 
true of the late or main crop produced in the Northern States. This 
crop is usually in constant danger of exposure to freezing tempera- 
ture from just before it is harvested until it reaches the consumer. 
There are two general classes of frost-injured potatoes — ^those frozen 
solid and subject to collapse immediately on thawing and those that 
show evidence of injury only on being cut open a few hours after 
being warmed. The first class of injured potatoes is easily identified 
by the soft, wet condition that develops on thawing. This type of 
injury is due to the potato being subjected to freezing at very low 
temperatures or to prolonged exposure at higher freezing tempera- 
tures. The second type of injury is not apparent on superficial ex- 
amination. It is due to exposure to temperatures just below the 
freezing point or to a very low temperature for a short time. If after 
the potato has been thawed a few hours it is cut open, evidences of 
this type of freezing injury are apparent by the presence of vascular 
discoloration of three types or a combination of any of the three. 

1 This bulletin gives the results of a portion of the work carried on under the project 
" Factors affecting the storage life of vegetables." 
11310°— 21 



%(l^ mJ/' 



2 BULLETIN 916, U. S. DEPARTMENT OE AGRICULTURE. 

PHYSIOLOGY OF THE FREEZING PROCESS. 

The frost necrosis of potato tubers is described by Jones, Miller, 
and Bailey (4)^ as being characterized by three types, viz, ring, net, 
and blotch. In the ring type only the vascular ring, an area near the 
surface, is discolored. In the net type threadlike areas radiating 
from the center are discolored. The third type is marked by irregular 
blotched areas. Ordinarily no further change takes place after the 
injured potato thaws, although in the most severe cases the tubers 
soon begin to break down if kept warm. 

Frost injury seems to be the result of actual ice formation within 
the i^otato tissue. Abbe (1) , in his investigations of the effect of freez- 
ing upon plant tissue, considers that as the tissue cools water exudes 
from the cells into the intercellular spaces. After sufficient under- 
cooling this water freezes. The concentrated sap remaining in the 
cells will not freeze until cooled below the freezing point of water. 
On thawing, this intercellular water escapes by transpiration and the 
plant wilts. Goppert (3) and Sachs (6) both observed the presence 
of ice within the cells and the intercellular spaces of pla-nt tissue. 
However, Sachs found crystals of ice usually present in the inter- 
cellular sjDaces. Miiller-Thurgau (5) was one of the first to publish 
(1880) upon frost injury of potatoes in his investigations upon the 
freezing point of plant tissue. The phenomena of undercooling were 
investigated by him to a considerable extent. He showed that plant 
tissue required undercooling below the true freezing point before 
actual freezing commences. He also showed that the expressed sap of 
the potato tuber freezes at a higher temperature than the living tis- 
sue. The ultimate freezing point of potatoes is placed at 30.21° F. by 
Miiller-Thurgau and between 28° and 26° F. by Appleman (2). 
Vaughan and Miller (T) state that " actual freezing does not begin 
in potatoes until the temperature drops below 28° F." Jones, Miller, 
and Bailey (4) place the freezing point at 28.4° F. 

The author of an unsigned article in the Potato Magazine (8) states 
that exposure at 30° F. for 9 or 10 hours or at 16° F. for 1 hour 
will develop signs of frost necrosis. In results published by Wright 
and Harvey (9) the freezing point varied from 29.67° to 28.13° F. 
according to variety and season. In this investigation the freezing 
points of 18 different varieties were determined. The freezing 
points varied in different varieties and according to family groups. 
The early and medium-early potatoes froze at a higher point than 
the purely late varieties. The subject of undercooling was discussed. 
It was stated that potatoes, since their sap consists of salts, sugar, and 
other soluble material, freeze at a lower temperature than pure water. 

2 The serial numbers in parentheses refer to " Literature ci ted " at the end of this 
bulletin. -■ — •"-"^ — ' " ^ 



LlSRAI^V'Cf CONGRESS 



•V^' 



JUN2lt9^1 

DOCUMENTS DIVISION 



<V' 



FREEZING INJURY TO POTATOES WHEN UNDERCOOLED. 



It was shown that potatoes could be imdercooled several degrees 
below the true freezing point without actual freezing taking place. 
In addition, it was shown that when once cooled below the true freez- 
ing point the process of undercooling can be terminated at any point 
by inoculating the tuber. The term "inoculation"^ is used in this 
connection to indicate any disturbance which will start crystalliza- 
tion or ice formation. Following inoculation the temperature of the 
potato rises quickly to the true freezing point, accompanied by the 
formation of ice within the tissue. Inoculation is caused b}^ some me- 
chanical disturbance, such as a jolt or jar sufficient to cause freezing 
to commence. It is a well-known fact that water, for example, can 
be undercooled below its freezing point. After a certain amount of 
undercooling actual freezing will result from any slight disturbance 
or by the addition of a crystal of ice. 

In the potato, inoculation is readily accomplished by striking or 
jarring the undercooled tuber. The degree of force necessary to 
accomplish this is determined by the extent of undercooling. It 
seems that a potato can be undercooled to such a point that the 
slightest perceptible jar will instantly inoculate it. Water on the 
surface or sap from a bruise will prevent a potato from reacliing the 
degree of undercooling it would attain if dry, since this free moisture 
will freeze at a higher temperature than the living tissue and will in 
this way inoculate the entire potato. Miiller-Thurgau (5) and Jones, 
Miller, and Bailey (4) found that a freshly cut potato will freeze 
more quickly ; that is, it will not undercool as far as a dry potato. 

Miiller-Thurgau was of the opinion that the undercooling point 
varies with the air temperature to which potatoes are exposed. He 
attempted to justify this view by data. In one instance freezing oc- 
curred in two hours at an exposure of 23.9° F. and in another in five 
hours at an exposure varying from 15.8° to 10.7° F. Jones and his 
colleagues (4), commenting on these results, state that possibly the 
earlier freezing at the higher undercooling temperature was due to a 
more rapid fall in temperature in this case. Data submitted by them 
and also in Department Bulletin 895, heretofore cited (9), indicate 
that the degree of undercooling is dependent for one thing upon the 
rate of fall of temperature. It is also entirely possible that a sudden 
unnoticed jar might have caused earlier freezing where the under- 
cooling temperature was higher. In Department Bulletin 895 (9, 
p. 6) a diagram is presented in which results are illustrated when two 
potatoes were exposed to different temperatures. One potato ex- 

3 It is consideaed that the term " inoculation " is rightly used here, as it is directly 
comparable to the process in physical chemistry, known by the same term, in which 
crystallization is started in a concentrated solution by adding a crystal of the solute or 
when freezing is started by adding a crystal of ice to pure water that has been 
undercooled. 



4 BULLETIN 916, U. S. DEPARTMENT OF AGRICULTURE. 

posed to a temperature of 8.7° froze in 6 minutes when the actual 
internal temperature had reached 23° F. The other potato was 
exposed to 15.8° and froze in 12.5 minutes when the internal tem- 
perature had reached 20.2° F. The actual freezing point of both 
potatoes was 29.15° F. This substantiates the results obtained by- 
Jones, Miller, and Bailey (4) . They cite an instance of a tuber freez- 
ing in 80 minutes when undercooled to 24.44°, while another exposed 
to a slowly diminishing temperature did not freeze until 13.1° F. 
was reached. Another specimen froze in 40 minutes when under- 
cooled to 26.4° while exposed to a rapidly diminishing tempera- 
ture which had reached 12.2° F. They state that, in general, po- 
tatoes do not freeze until exposed to 26.6° F. or lower. To judge 
from the results submitted in this report it is possibly safe to state 
that except in the case of accidental inoculation the degree of under- 
cooling at which freezing occurs depends upon the variety, the 
rapidity with which undercooling progresses, and the length of ex- 
posure to a given degree of undercooling. 

PLAN OF THE WORK. 

In order to apply to the handling of potatoes the knowledge of the 
process of undercooling and freezing already gained, a series of ex- 
periments was conducted at the Arlington cold-storage plant of the 
Bureau of Plant Industry. It has been noted by potato growers and 
shippers that sometimes one lot of potatoes while being handled or 
otherwise disttirbed in transit will freeze when another left undis- 
turbed at the same temperature remained uninjured. Some apple 
handlers are very careful not to jar or jolt frosted apples, because 
they say it will cause them to bruise and rot. It is a practice among 
some onion growers to store their onions through very severe winter 
weather in mows or buildings between layers of hay where they are 
undisturbed during the winter. These persons will often guard 
against loose doors or shutters which can be slammed by the wind, 
as this is liable to cause the onions to freeze and subsequently to rot. 
Numerous instances are reported where potatoes have been exposed 
for hours or even days to temperatures below their freezing point 
without injury. Miiller-Thurgau reported having stored potatoes 
at 32° to 26.6° F. for two weeks without injury. Of course, internal 
frost injury may have been present without its being detected unless 
the potatoes were cut open and examined. 

In the present work seven standard varieties of potatoes, all true 
to name and grown under the same conditions by the Bureau of 
Plant Industry at the Aroostook Farm of the Maine Agricultural 
Experiment Station, were used. These varieties were the Triumph, 
Irish Cobbler, Spaulding No. 4, American Giant, Eural New Yorker, 



FREEZING INJURY TO POTATOES WHEN UNDERCOOLED. 



Russet Eiiral, and Green Mountain. They were all held at 40° F. 
previous to experimental use. 

Storage tests on the seven varieties held at temperatures below the 
true freezing points were conducted in order to correlate, if possible, 
the freezing points (Table I) of these varieties as determined by the 
thermoelectric method described by Wright and Harvey (9) and the 
actual freezing of the different varieties in storage. These tests were 
conducted at 28° and 25° F. The higher temperature was chosen 
because it represented about the minimum degree of undercooling to 
which the potatoes could be subjected, since it was just below the 
freezing points of most of the varieties used. The lower tempera- 
ture was chosen because it was far enough below the freezing points 
of all varieties to represent a definite degree of undercooling at which 
freezing quickly follows a very light inoculation. When the potatoes 
were selected for the freezing tests they were placed in small wooden 
baskets holding from 8 to 10 specimens, in order that they might cool 
down without much delay. At the conclusion of each test the potatoes 
were removed, to be held at ordinary room temperature for about 24 
hours before being cut longitudinally for examination. If cut imme- 
diately after being removed from stora,ge, no evidence of freezing 
injury will be apparent unless they have been frozen solid. Unless 
specially noted, all injury reported is of the vascular type, as de- 
scribed by Jones, Miller, and Bailey. 

Table I. — Freezing points of seven varieties of potatoes. 



Variety. 


Freezing 
point. 


Variety. 


Freezing 
point. 


Triumph 


"F. 

29.04 
29.66 
29.33 
29.64 


Rura 1 New Yorker 


°F. 
28.70 


Irish Cobbler 


Russet Rural 


28.32 


Spaulding No. 4 


Green Mountain 


28.50 


American Giant 











Tables II and III show the results obtained from these tests at 28° 
and 25° F. The data presented in Table II show that potatoes did 
not freeze to any serious extent when exposed to 28° F. for many 
hours. In experiment No. 1 two specimens of each variety were held 
seven hours, one specimen of Rural New Yorker being injured. In 
experiment No. 2 twenty specimens of each variety were held 24 hours. 
Onl}^ two varieties showed injury. These included two specimens of 
Irish Cobbler and one specimen of the American Giant. In the third 
experiment two specimens of each variety were held 48 hours, one 
Rul-al New Yorker being injured. In experiment No. 4 half -bushel 
lots of Triumph, Spaulding No. 4, American Giant, and Russet Rural 
varieties were held in bags for 48 hours with no injury. In experi- 
ment No. 5 eleven specimens of each variety were held for TO hours. 



BULLETIN 916, U. S. DEPAKTMENT OF AGRICULTURE. 



Serious injury was apparent in this experiment. Injury occurred to 
five specimens of Triumph, six of Irish Cobbler, five of Spaulding 
No. 4, four of American Giant, one of Eural New Yorker, two of 
Russet Rural, and one of Green Mountain. The most serious injury 
occurred in the early-maturing varieties. In experiment No. 6 six 
tubers of each variety were held 96 hours, with injury occurring to 
two specimens of Triumph, two of Irish Cobbler, two of American 
Giant, and one of Russet Rural. The Spaulding No. 4, Rural New 
Yorker, and Green Mountain varieties suffered no injury. The aggre- 
gate injury in this experiment was 17 per cent, while in experiment 
No. 5 the injury totaled 31 per cent. 

Table II. — Freezing injury to seven varieties of potatoes held at 28° F. for 

different lengths of time. 



Variety. 



Triumph 

Irish Cobbler 

Spaulding No. 4 

American Giant 

Rural New Yorker. 

Russet Rural 

Green Mountain . . . 

Total 



Number of experiment and duration of exposure. 



No. 1,7 
hours. 


No. 2, 24 
hours. 


No. 3, 48 
hours. 


No. 4, 48 
hours. 


No. 5, 70 
hours. 


No. 6, 96 
hours. 


Num- 
ber of 


Num- 
ber 


Num- 
ber of 


Num- 
ber 


Num- 
ber of 


Num- 
ber 


Quan- 
tity of 


Num- 
ber 


Num- 
ber of 


Num- 
ber 


Num- 
ber of 


Num- 
ber 


speci- 
mens. 


in- 
jured. 


speci- 
mens. 


in- 
jured. 


speci- 
mens. 


in- 
jured. 


speci- 
mens. 


in- 
jured. 


speci- 
mens. 


in- 
jured. 


speci- 
mens. 


in- 
jured. 


2 
2 
2 

2 

2 
2 
2 





1 





20 
20 
20 
20 
20 
20 
20 



2 

1 





2 
2 
2 
2 
2 
2 
2 





(; 
1 





Bush, 
h 





^^ 


5 
6 
5 
4 
1 
2 
] 


6 
6 
6 
6 
6 
6 
6 


2 
2 


1 

2 
h 









2 


1 











14 


1 


140 


3 


14 


1 


2 





77 


24 


42 


7 



In no case at 28° F. were the potatoes frozen solid. A superficial 
examination would not have revealed any evidence of freezing injury 
in any of these varieties. It is true that during subsequent storage, 
however, these internally injured specimens are the first to break 
down, although these potatoes do not show injury so far as their 
external appearance is concerned and would be as salable in the 
ordinary market as the uninjured ones. After a few days at ordinary 
room temperature they are quite unfit for food. While no serious 
injury occurred till the potatoes were held at least 70 hours, isolated 
cases of freezing injury did occur even when potatoes were held only 
7 hours. It is difficult to explain these isolated cases. For some 
unknown reason certain individuals seem to bear much less under- 
cooling than others of the same variety. Similarly, certain varieties 
bear less undercooling than other varieties. There is a certain va- 
rietal difference shown here. The Irish Cobblers seem to be the 
first to succumb to freezing injury, folloAved by the Triumph, Ameri- 
can Giant, Spaulding No. 4, and Russet Rural varieties. The Rural 
New Yorker and Green Mountain varieties are the most resistant. 



FREEZING INJURY TO POTATOES WHEN UNDERCOOLED. 



Table III. — Freezing injury to seven varieties of potatoes held at 25° F. for 

different lengths of time. 



Variety. 



Triumph 

Irish Cobbler 

SpauldingNo.4... 
American Giant.. . 
Rural New Yorker 

Russet Rural 

Green Mountain . . 

Total 



Niunber of experiment and duration of exposure. 



No. 1, 5 hours. 



Number 
of speci- 
mens. 



14 



Number 
injured 



No. 2, 19 hours. No. 3, 24 hours 



Number 
of speci- 
mens. 



42 



Number 
injured. 



Niunber 
of speci- 
mens. 



77 



No. 4, 43 hours. 



Nimiber 
injured 



36 



Number 
of speci- 
mens. 



Number 
injured. 



21 



o2 
1 
3 


b3 
1 



11 



a Frozen solid. 



b Two were frozen solid. 



In Table III are shown the results obtained when the different 
varieties were held at 25° F. In experiment No. 1 two specimens 
of each variety were held 5 hours with no evidence of injury. In 
experiment No. 2 six specimens of each variety were stored 19 hours, 
with injury to two Irish Cobblers, two American Giants, one Rural 
New Yorker, one Russet Rural, and one Green Mountain. It is 
worthy of note that the Triumph and Spaulding No. 4 varieties, 
which according to Table I have a comparatively high freezing point, 
did not show injury in this experiment. In experiment No. 3 eleven 
specimens of each were held 24 hours, with serious consequences to 
most of the varieties. Injury was found in five Triumphs, six Irish 
Cobblers, eight Spaulding No. 4, six American Giants, five Rural New 
Yorkers, two Russet Rurals, and four Green Mountains. The total 
injury was 47 per cent. In experiment No. 4 three tubers of each 
variety were held 43 hours, with injury to one Triumph, two Irish 
Cobblers, one Spaulding No. 4, three American Giants, three Russet 
Rurals, and one Green Mountain. Tubers of the Rural New Yorker 
were not injured; one specimen of Triumph, one of Irish Cobbler, 
and two of the Russet Rural varieties were frozen solid. The total 
injury in this experiment was 52 per cent. 

Generally speaking, when potatoes of the seven varieties were held 
for varying lengths of time at 28° or 25° F. they did not freeze in 
any definite order or with relation to their freezing points. At 28° F. 
the early-maturing varieties with the higher freezing points possibly 
showed more freezing injury. 

INOCULATION OF UNDERCOOLED POTATOES. 

Attention is called to the fact that in the experiments just de- 
scribed the potatoes were purposely held entirely undisturbed, as it 
had been found that potatoes when undercooled are liable to freeze 



8 BULLETIN 916, U. S. DEPARTMENT OE AGRICULTURE. 

when jarred. It is considered highly important to call the attention 
of growers and shippers to this point, since under some circumstances 
it would be entirely possible that a lot of potatoes or even other prod- 
ucts might be undercooled considerably below the freezing point and 
escape injury if they remained undisturbed till their temperature was 
above the freezing point. If it should be found necessary to move 
such a lot in order to protect them from further lowering of the 
temperature, this should be very carefully done, having in mind the 
danger of freezing as the result of jarring or jolting. These state- 
ments seem justified and are supported by the results brought out by 
the following experiments, which were conducted to determine the 
amount and character of the disturbance necessary to cause freezing 
when different varieties of potatoes are held at temperatures between 
28° and 25° F. The same seven standard varieties before named were 
used in these experiments. 

EXPERIMENT NO. 1. 

Duplicate lots of potatoes, consisting of a single specimen of each 
variety, were held between 26° and 27° F. for 5 J hours. Then each 
specimen in one lot was dropped from a distance of 4 feet to the 
concrete floor. After IT hours from the time they were dropped, 
both lots were removed and held at ordinary room temperature 
several hours before they were cut open. In the check lot only the 
Irish Cobbler and the American Giant were injured. All specimens 
in the treated lot were not only badly bruised, as might be expected 
from the rough treatment, but they were all frost injured, as was 
shown by extensive blackened vascular areas of the blotch type. 
(PI. I.) None of the potatoes were frozen hard enough to cause 
collapse of the tissues. 

EXPERIMENT NO. 2. 

Duplicate lots of one potato of each variety were held at 28° F. for 
22 hours. One lot was then dropped as before and left for three 
hours. Both lots were then removed. As before, at the lower tem- 
perature all the dropped potatoes were bruised and showed extreme 
freezing injury. In the undisturbed check lot only the Irish Cobbler 
was injured. 

EXPERIMENT NO. 3. 

Duplicate lots of one potato of each variety were placed at 28° F. 
for 24 hours. Each specimen from one lot was dropped from a dis- 
tance of 2 feet, and after another 24 hours all were removed. On 
the usual examination it was found that these potatoes were badly 
bruised and showed extensive injury, as before, except the American 
Giant and Spaulding No. 4 varieties, which showed no frost injury 
and were not bruised. 



Bui. 916, U. S. Dept. of Agriculture. 



Plate I, 



>s. 





Freezing Injury to Potatoes Artificially Produced. 

The potatoes were undercooled at 25° F. for 24 hours, then inoculated by dropping 
6 inches. In one hour after inoculation they were removed to a warm room. 
One specimen (Irish Cobbler, fig. 1) shows only sUght freezing injury of the ring 
type. The other specimen (American Giant, fig. 2) shows severe injurj' of the 
ring and blotch typo. 



rREEZIN"G INJURY TO POTATOES WHEN UNDERCOOLED. 



9 



EXPERIMENT NO. 4. 

Duplicate lots of two potatoes of each A^ariety Avere held at 28° F. 
for 24 hours, after which all of one lot were dropped 1 foot. After 
another 24 hours all were remoA^ed. Examination showed no bruis- 
ing; hoAveA^er, most of the potatoes AAere frozen. The results are 
shoAA^n in Table IV. 

Table IY. — Freezinfj injury to potatoes of seven different varieties undercooled 

at 28° F. and dropped 1 foot. 



Variety. 



Dropped. 



Injured. Uninjured. 



Triumph 2 

Irish Cobbler, 2 

Spaulding No. 4 

American Giant 2 

Rural New Yorker 2 

Russet Rural 2 

Green Mountain 

Total 10 



Check. 



Injured. Uninjured. 



13 



EXPERIMENT NO. 5. 

A total of 24 potatoes of the Eural Xew Yorker A^ariety were placed 
at 28° F. After 24 hours 12 were dropped 12 inches. In no case was 
a potato bruised or eA^en the skin broken. In 4 hours both lots were 
remoA^ed. Examination showed all of the lot that was dropped to 
be injured by freezing. None of the check lot Avas injured. 

EXPERIMENT NO. 6. 

This experiment was conducted to ascertain, if possible, the mini- 
mum distance from which an undercooled potato may fall and still 
succumb to freezing injury. FiA^e lots of 12 potatoes of the Rural 
New Yorker variety were held at 28° F. for 18 hours. Then lots 
were dropped 2, 4, and 6 inches, and in one lot each individual was 
struck with a pencil. After seven hours all were remoA^ed. Examina- 
tion showed that only the potatoes that were struck with the pencil 
were injured. 

EXPERIMENT NO. 7. 

Lots of three potatoes of each variety were dropped, as in experi- 
ment No. 6, with the exception that each potato was dropped six times 
instead of once. The results are shoAvn in Table Y. Practically the 
same amount of freezing injury was produced in all dropped pota- 
toes without relation to the length of the fall. The injury produced 
was of the blotch type. It may be stated here that the potatoes used 
in this experiment were i^urposely selected and weighed, so as to haA'e 
both large and small specimens in each lot, thus varying the force of 



10 



BULLETIN 916, U. S. DEPARTMENT OF AGRICULTURE. 



the blow caused by the fall. There was apparently no relation be- 
tween the weight of the ]3otato and the freezing injury. In addition 
to the lots already described in this experiment, an additional lot of 
two of each variety was treated by striking each potato sharply with 
a pencil once, with the results shown in Table Y. It is difficult to 
state why it was necessary to drop the potatoes several times from 
2, 4, and 6 inch heights to produce frost injury, while one sharp blow 
with an ordinar}^ pencil results in serious injury. The possible expla- 
nation is that the blow from the pencil caused a more violent concus- 
sion in a smaller area, from which the whole potato was inoculated. 

Table V. — Freezing injury to potatoes of seven different varieties undereooled 
at 28° F. and inoculated in different ways. 



Variety. 


Dropped 
2 inches. 


Dropped 
4 inches. 


Dropped 
6 inches. 


Struck with 
pencil. 


Check. 


In- 
jured. 


Unin- 
jured. 


In- 
jured. 


Unin- 
jured. 


In- 
jured. 


Unin- 
jured. 


In- 
jured. 


Unin- 
jured. 


In- 
jured. 


Unin- 
jured. 


Triumpli 


3 
3 
1 
3 
3 
3 
3 




2 






3 
3 
3 
3 
3 
3 
2 









1 


3 
3 
2 
3 
3 
2 
2 




1 



1 
1 


1 

1 
1 
2 
2 
2 
2 


1 

1 
1 









1 




1 


3 


Irish Cobbler 

Spaulding No. 4 

American Giant 

Rural New Yorker. . 

Russet Rural 

Green Mountain 


3 
3 
2 
3 
3 
2 


Total 


19 


2 


20 


1 


18 


3 


11 


3 


2 


19 







EXPERIMENT NO. 8. 

Two lots of one specimen of each variety of potato were held at 
28° F. for 24 hours, after which each potato of one lot was repeatedly 
bounced on the floor for about 10 seconds by dropping it from a height 
of not over 1 inch. After another 24-hour period all were removed. 
While none of the bounced specimens showed bruising or mechanical 
injury, they were all frost injured. The specimens of American Giant 
and Green Mountain froze solid, with resultant complete collapse on 
thawing. Of the check lot the Spaulding No. 4 and the Green Moun- 
tain varieties showed slight freezing injury. 

EXPERIMENT NO. 9. 

Lots of three potatoes of each variety were held at 25° F. for 18 
hours, after which they were dropped once from distances of 6, 4, and 
2 inches. After another hour all were removed. Examination 
showed that all dropped potatoes were injured, while in the check 
lot frost injury resulted to only one each of the Irish Cobbler, Ameri- 
can Giant, Russet Rural, and Green Mountain varieties. 

Several experiments were conducted to test the rate at which the 
temperature of a potato will fall when exposed to a temperature be- 
low its freezing point. Figure 1 illustrates a typical set of results. 



FREEZmG INJURY TO POTATOES WHEN UNDERCOOLED. 11 

A Triumph potato was suspended in a metal container 18 inches long 
by 4 inches in diameter, which was in turn immersed in a brine tank. 
Thermoelectric couples were located at the center and near the surface 
of the potato; also one was suspended in the air 1 inch from the 
potato. Periodical readings were made from the time the temperature 
of the potato was near 40° F. The temperature of the atmosphere 
surrounding the potato gradually fell from 27.2*^ to 25.6° F. The 
temperature of the potato at the beginning was 40.8° at the surface 




^^ /5 JO 4S 60 rs &0 /OS" /£V A3S /50 /65- /60 /9S 2/0 2eS 3^i7 eSS 2/^0 ^SS 30C 

Fig. 1. — Temperatures at the center and near the surface of a Triumph potato as it 
undercooled to between 26° and 27° P. At this point it was inoculated by tapping it 
sharply with a pencil. As the potato froze, the temperature rose. When all the heat 
was liberated, the temperature of the potato reached 28.7° F. and remained several 
minutes. The temperature of the air bath to which the potato was exposed is also 
slaown. Note the accompanying rise in the temperature of the surrounding air as 
the potato froze and liberated heat. 

and 39.9° F. at the center. The difference in temperature at the 
surface and center gradually diminished until at 240 minutes there 
was a di:fference of only 0.4 of a degree. After this the difference 
became somewhat greater. In 285 minutes the temperature fell from 
40.8° and 39.9° F. at' the surface and center, respectively, to 26.7° and 
26.2° F. At this point the potato was tapped sharply with a pencil, 
and freezing commenced. The temperature at both points began to 
rise as heat was liberated, due to crystallization of the water. In five 
minutes the temperature at the center rose to 30.2° F., and at the 
surface to 29.5° F. In 10 minutes the temperature at both points was 
practically the «>ame, 28.7° F. This represents the freezing point. 



12 



BULLETIN 916, U. S. DEPARTMENT OF AGPJCULTURE. 



The temperature of the air surrounding the potato at the time freez- 
ing took place rose from 25.6° to 26.6° F., due to heat liberated by the 
potato. 

SUMMARY OF RESULTS OBTAINED BY DROPPING UNDERCOOLED POTATOES. 

When undercooled at 28° F. and dropped 4 feet and 2 feet, the 
potatoes were severely bruised and showed extreme freezing injury 
of the vascular type. When dropped 1 foot, no bruising resulted, but 
all varieties froze except the Green Mountain and Spaulding No. 4. 
The type of injury was a faint blotch. Wlien dropped 6, 4, and 2 
inches once, no freezing injur}^ resulted ; but when drop]3ed six times 
from these heights, severe injury followed. When bounced several 
times by dropping 1 inch, severe injury followed; in some cases the 
potatoes were frozen solid. When undercooled at 25° F., dropping 
once from 6, 4, and 2 inches caused injury. 

Some further experiments were undertaken to determine what 
other forms of disturbances will inoculate undercooled potatoes. 
Duplicate lots of 12 specimens each of the Rural New Yorker variety 
were placed at 25° and 28° F. for 24 hours. At each temperature 
one lot was placed in the direct draft from an electric fan, while the 
other lot was protected from the draft to act as a check. At 25° F. 
10 of the 12 potatoes in the draft showed injury; also 3 of the 
check lot were injured. At 28° F. no injury was found in either lot. 
A possible explanation for the fact that the potatoes froze when 
exposed at 25° F. to a strong current of air is that they undercooled 
much more rapidly than the lot not in the draft. 

Two lots of six specimens each of Rural New Yorker were held at 
28° F. for 24 hours, then wet and removed after another 24 hours. 
All of the lot that were wet were injured, while three of the lot that 
were not wet also were injured. In the next experiment two lots 
consisting of two of each variety were held at 28° F. for 24 hours ; 
they were then wet and held for another 24 hours. The results ob- 
tained on examination are shown in Table VI. 

Table VI. — Fi^eezing injury to potatoes of seven different varieties Jield at 
28° F. for 24 hours and then wet ivhile undercooled. 



Variety. 


Wet. 


Check. 


Injured. 


Uninjured. 


Injured. 


Uninjured. 


Triumph 


2 
2 

1 
2 
2 
1 



2 
1 


1 






1 




2 


Irish Cobbler 


2 


Spaulding No. 4 


2 


American Giant 


2 


Rural New Yorker 


1 


Russet Rural 


2 


Green Mountain 


2 






Total 


10 


4 


1 


13 







a Faint blotch. 



FREEZmG INJURY TO POTATOES WHEN UNDERCOOLED. 13 

The injury to these potatoes was not serious. All affected ones 
showed only a faint blotching. In the next experiment a mixed lot 
of 12 potatoes was held at 28° F. for 72 hours while supporting a 
100-pound bag of sand. No evidence of injury was apparent. 

An attempt was made to show that potatoes when undercooled are 
liable to be injured by ordinary handling. Duplicate half-bushel 
lots of potatoes representing four varieties, viz, Triumph, Russet 
Rural, Spaulding No. 4, and American Giant, were put together in 
four bags and held at 28° F. for 24 hours. Then one lot of two bags 
was rolled across the floor for a distance of about 30 feet. After 
24 hours all were removed. Examination showed the results pre- 
sented in Table VII. 

Table YU.— Freezing injury to potatoes of four different varieties held at 28° F. 
for 24 hours and then rolled 30 feet in a Mg while undercooled. 



Variety. 



Triiimph 

Spaulding No. 4, 
American Giant 
Russet Rural.... 



Rolled. 



Per cent 
injured. 
10 


20 
50 



Check. 



Per cent 
injured. 



A similar experiment was carried out in which two lots of 12 of 
each of the seven varieties were placed in two bags and held at 28° F. 
for 24 hours. One was then rolled about 50 feet. After another 
period of 24 hours all were removed and held for examination. The 
results obtained are shown in Table VIII. 



Table VIII. — Freezing injury to potatoes of seven 
28° F. for 24 hours and then rolled 50 feet in a 


different varieties held at 
bag while undercooled. 




Rolled. 


Check. 


Variety. 


Number 
injured. 


Number 
uninjured. 


Number 
injured. 


Niunber 
uninjured. 


Triumph 


5 

12 

2 
12 
8 
7 


7 

12 
10 

4 
5 











12 


Irish Cobbler 


12 


Spaulding No. 4 


12 


American Giant 


12 


Rural New Yorker 


12 


Russet Rural 


12 


Green Mountain 


12 






Total 


46 


38 





84 







A distinct varietal difference is shown here. In neither experiment 
did the Spaulding No. 4 variety show injury. The American Giant 
was also less susceptible than the rest of the varieties. The Irish 
Cobbler, Rural New Yorker, and Russet Rural showed the greatest 
percentage of injury. In these experiments the potatoes did not 
receive the amount of jolting or rough handling that they might be 



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